This invention relates to a cross polarization interference compensating or cancelling device for use in an orthogonal polarization communication system in which use is made of two orthogonally polarized signals of a common frequency.
Attention is recently directed in microwave radio communication to an orthogonal polarization communication system in which effective use is made of a first-polarization and a second-polarization signal as two orthogonally polarized signals of a common frequency. Into the two orthogonally polarized signals, differently polarized components are inevitably introduced due to heterogeneity which is given to a transmission medium by rain and the like. In other words, each of the two polarization signals are subjected to a cross polarization interference. Consequently, various cross polarization compensating devices are proposed for use in compensating for the interference.
One of such cross polarization compensating devices is described in a paper contributed by Hiroyuki Ohtsuka and two others under the title of "The SBS Control Algorithm of Cross Polarization Interference Canceller on Digital Radio Systems.revreaction. to the Transactions of the IEICE, Volume E 73, No. 3 (March 1990), pages 401 to 408. In the manner which will later be described more in detail, the cross polarization interference compensating device of Ohtsuka et al comprises a first equalizer for equalizing a first-polarization signal into a first equalized signal devoid of a first intersymbol interference component which is superposed on the first-polarization signal. A second equalizer equalizes a second-polarization signal into a second equalizer output signal exempted from a second intersymbol interference component superposed on the second-polarization signal. Supplied with the second-polarization signal, a first cross polarization interference canceller produces a first canceller output signal related to a first cross polarization interference component which is superposed on the first-polarization signal by the second-polarization signal. Supplied with the first-polarization signal, a second cross polarization interference canceller produces a second canceller output signal concerned with a second cross polarization interference component which is superposed on the second-polarization signal from the first-polarization signal.
A first adder adds the first canceller output signal to the first equalizer output signal to produce a first sum signal in which the first-polarization signal is exempted from the first intersymbol and cross polarization interference components. Similarly, a second adder produces a second sum signal. Supplied with the first sum signal, a first decision circuit produces a first decided signal representative of a symbol sequence carried by the first-polarization signal. Likewise, a second decision circuit produces a second decided signal representative of the symbol sequence carried by the second-polarization signal.
The conventional cross polarization interference compensating device is, however, defective in that the intersymbol and the cross polarization interference components are not reliably removed from the first and the second sum signals when become large. The decision circuits may misjudge the symbol sequence under the circumstances.